The crude oil which has accumulated in subterranean reservoirs is recovered or produced through one or more wells drilled into the reservoir. Initial production of the crude oil is accomplished by "primary recovery" techniques wherein only the natural forces present in the reservoir are utilized to produce the oil. However upon depletion of these natural forces and the termination of primary recovery, a large portion of the crude oil remains trapped within the reservoir. Also many reservoirs lack sufficient natural forces to be produced by primary methods from the very beginning. Recognition of these facts has led to the development and use of many enhanced oil recovery techniques. Most of these techniques involve injection of at least one fluid into the reservoir to produce an additional amount of the crude oil therefrom. Some of the more common method are water flooding, steam flooding, miscible flooding, CO.sub.2 flooding, polymer flooding, surfactant flooding, caustic flooding and in situ combustion.
Water flooding, which involves injection of water into the subterranean oil reservoir for the purpose of displacing the crude oil from the pore spaces of the reservoir rock towards the producing wells is the most economical and widely used of the enhanced oil recovery method. Nevertheless water does not displace oil with high efficiency because of the immiscibility of water and oil and because of the high interfacial tension between them.
Surfactant flooding involves the addition of one or more surface active agents, or surfactants, to the water flood for the purpose of minimizing the water flooding problems mentioned above. This has been an area of active interest in the art of enhanced oil recovery methods for many years. For example in 1941, U.S. Pat. No. 2,233,381 disclosed the use of polyglycol ether as a surfactant which increases the capillary displacement efficiency of an aqueous flood. U.S. Pat. No. 3,302,713 discloses the use of petroleum sulfonates as effective surfactants in oil recovery operations. Other surfactants proposed for use in oil recovery processes include alkylpyridinium salts, alkyl sulfates, alkylaryl sulfates, ethoxylated alkyl or alkylaryl sulfates, alkyl sulfonates, alkylaryl sulfonates and quaternary ammonium salts.
While the above surfactants may be effective under ideal conditions, there are problems concerned with the use of each in most petroleum reservoirs. Some of the most serious problems arise from the effects of reservoir fluid salinity on the injected surfactant, the most common being precipitation and resultant loss of the surfactant. Mixtures of different types of surfactants, such as anionic and non-ionic, are employed in many prior art techniques to achieve a higher salinity tolerance for the mixture. However, even this is not entirely satisfactory because as the mixture is driven through the formation, one of the components is often preferentially adsorbed to the mineral grains in the formation matrices, causing a change in the relative concentration of the surfactant components and resulting in a failure to maintain effective salinity tolerance.
Another serious problem concerns the vertical conformance efficiency of a surfactant flooding operation. Most reservoirs display significant permeability variations throughout their volumes causing fluids to flow preferentially through the high permeability sections, leaving portions of the reservoir with lower permeabilities essentially isolated from the effects of the injected fluids. The solution of this problem is thought to lie in selectively plugging the higher permeability streaks in order that the injected fluids be then forced into the previously unswept lower permeability portions of the reservoir. Several solutions of this problem have been proposed, but none have proved to be entirely successful. It is felt that most of these proposed solutions only affect a region relatively close to the injection well bore. The most effective process would appear to be a technique that is continuous with the entirely of the surfactant injection cycle and would contact all but the very lowest permeability elements of the total reservoir volume.
It can be readily seen that there remains a substantial need for a surfactant flooding process that will perform effectively in a high salinity reservoir environment while displaying a high vertical conformance efficiency.